- TUTORIALS: PHOTOSHOP LEVELS -

Levels is a tool in Photoshop and other image editing programs which can
move and stretch the brightness levels of an
image histogram. It has the power to adjust brightness, contrast,
and tonal range by specifying the location of complete black, complete white,
and midtones in a histogram. Since every photo's histogram is unique,
there is no single way to adjust the levels for all your photos. A proper
understanding of how to adjust the levels of an image histogram will help you
better represent tones in the final image.

HOW IT WORKS

The levels tool can move and stretch brightness levels in a histogram using
three main components: a black point, white point and midtone slider.
The position of the black and white point sliders redefine the histogram's "Input
Levels" so they are mapped to the "Output Levels" (default is black (0) or white
(255), respectively), whereas the midtone slider redefines the location of middle
gray (128). Each slider is shown below as they appear in Photoshop's levels
tool, with added blue labels for clarity:

All examples below will use the levels tool on an
RGB histogram, although levels can also be performed on other types of histograms.
Levels can be performed on an individual color channel by changing the options
within the "Channel" box at the top.

ADJUSTING THE BLACK AND WHITE POINT LEVELS

When considering adjusting the black and white point levels of your histogram,
ask yourself: is there any region in the image which should be completely black
or white, and does the image histogram show this?

Most images look best when they utilize the full range dark to light which
can be displayed on your screen or in a print. This means that it is often
best to perform levels such that the histogram extends all the way from black
(0) to white (255). Images which do not extend to fill the entire tonal
range often look washed out and can lack impact. The image below was taken
in direct sunlight and includes both bright clouds and dark stone shadows--
an example of where there should be at least some regions that are portrayed
as nearly white or black. This histogram can be extended to fill the entire
tonal range by adjusting the levels sliders as shown:

Histogram Before Levels

Histogram After Levels

Lower Contrast

Higher Contrast

On the other hand, be wary of developing a habit of simply pushing the black
and white point sliders to the edges of the histogram-- without also paying
attention to the content of your image. Images taken in fog, haze or very
soft light often never have fully black or white regions. Adjusting levels
for such images can ruin the mood and make your image less representative of
the actual scene by making it appear as though the lighting is harsher than
it actually was.

One should also be cautious when moving the black and white point sliders
to the edge of the histogram, as these can easily clip the shadows and highlights.
A histogram may contain highlights or shadows that are shown with a height of
just one pixel, and these are easily clipped. This is often the case with
low-key images (see
histograms tutorial), such as the example shown below:

Histogram Before Levels

Histogram After Levels

No Pixel at Full Brightness

Stronger Highlights

Holding down the "ALT" key while dragging the black or white point slider
is a trick which can help avoid shadow or highlight clipping, respectively.
If I were to have dragged the highlight slider above to a point which was further
left (a level of 180 was used, versus 235 above), while simultaneously holding
down ALT, the image would have appeared as follows:

If the image is fully black while dragging a black or white point slider,
then no clipping has occurred. When the slider is dragged over where there
are counts on the histogram, the regions of the image which have become clipped
get highlighted as shown above. This can be quite useful because knowing
where the clipping will occur can help one assess whether this will actually
be detrimental to the artistic intent of the image. Keep in mind though
that clipping shown while dragging a slider on an RGB histogram does not necessarily
mean that region has become completely white-- only that at least one of the
red, green, or blue color channels has reached its maximum of 255.

ADJUSTING THE MIDTONE LEVEL

Moving the midtones slider compresses or stretches the tones to the left
or right of the slider, depending on which direction it is moved. Movement
to the left stretches the histogram to the its right and compresses the histogram
to its left (thereby brightening the image by stretching out the shadows and
compressing the highlights), whereas movement to the right performs the opposite.
Therefore, the midtone slider's main use is to brighten or darken the midtones
within an image.

When else should one use the midtone slider? Consider the following
scenario: your image should contain full black and white, and even though the
histogram extends to full black, it does not extend to white. If you move
the white point slider so that it reaches the edge of the histogram, you end
up making the image much brighter and overexposed. Using the midtone slider
in conjunction with the white point slider can help you maintain the brightness
in the rest of your image, while still stretching the highlights to white:

Histogram Before Levels

Histogram After Levels

Sky Not At Full Brightness

Stronger Highlights
Similar Overall Brightness

Note how the sky became more pronounced, even though the overall brightness
of the image remained similar. If the midtones tool were not used, the
image to the right would have appeared very overexposed. The same method
could be used to darken the shadows while maintaining midtones, except the midtones
slider would instead be moved to the left.

Note: Even though the midtones slider is always initially at 128, it is instead
shown as 1.00 to avoid confusion when the black and white points change.
This way, the midtones slider is always at 1.00 even when the other sliders
have been moved. The midtone "Input Level" number actually represents
the gamma adjustment, which can be thought of as a relative measure of the number
of levels on the sliders left to those on its right. Thus, values greater
than one mean there are more levels are to the slider's right, whereas values
less than one mean more levels are to its left.

ADJUSTING LEVELS WITH THE DROPPER TOOLS

The histogram levels can also be adjusted using the dropper tools, shown
below in red:

One can use the dropper tools on the far left and right to set the black
and white points by clicking on locations within the image that should be either
black or white, respectively. This is often not as precise as using the
sliders, because one does not necessarily know whether clicking on a given point
will clip the histogram. The black and white point droppers are more useful
for computer-generated graphics as opposed to photos.

Unlike the black and white point droppers, the middle dropper tool does not
perform the same function as the midtone slider. The middle dropper actually
sets the "gray point," which is a section of the image that should be colorless.
This is useful when there is a colorless reference object within your scene;
one can click on it with the dropper tool and removing color casts by setting
the
white balance. On the other hand, it is better to perform a white
balance on a RAW file
format since this reduces the risk of
posterization.

OTHER USES FOR THE LEVELS TOOL

The levels tool can be performed any type of image histogram in addition
to the RGB histograms shown above, including
luminance and color histograms. Performing levels on a luminance histogram
can be useful to increase contrast without also influencing color saturation,
whereas levels on a color histogram can change the color balance for images
which suffer from unrealistic color casts (such as those with an incorrect
white
balance).

Levels can also be used to decrease the contrast in an image by modifying
the "Output Levels" instead of the "Input Levels." This can be a useful
step before performing techniques such as
local
contrast enhancement since it avoids clipping (because this technique may
darken or brighten the darkest or brightest regions, respectively), or when
your image contains too much contrast.

PRECAUTIONS

Minimize use of the levels tool, as anything which stretches the image
histogram increases the possibility of
posterization.

Performing levels on a
luminance histogram can easily clip an individual color channel, although
this may also allow for darker and brighter black and white points, respectively.

Performing levels on an individual
color histogram or channel can adversely affect the color balance, so
color channel levels should only be performed when necessary or intentional
color shifts are desired.